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Atoms, bonding and types of reactions

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Electronic configuration of atoms (Part 2)

For Chromium, you would expect Cr: [Ar] 3d44s2, however it is actually Cr: [Ar] 3d54s1. Similarly for Copper one would expect Cu: [Ar] 3d94s2, however it is actually Cu: [Ar] 3d104s1.

Some people try to explain the break in pattern by claiming that half-filled or completely-filled d orbitals are more stable than a completely-filled s-orbital. For the purposes of A-level examinations this is a satisfactory explanation, however this is not actually the case! Continue to the Stretch & Challenge section if you wish to learn more.

Continuing from Chromium:

Cr: [Ar] 3d54s1

Mn: [Ar] 3d54s2

Fe: [Ar] 3d64s2

Co: [Ar] 3d74s2

Ni: [Ar] 3d84s2

Cu: [Ar] 3d104s1

Zn: [Ar] 3d104s2

Remember, electrons will fill available orbitals singly (spin aligned) before pairing up (spin paired). This is illustrated for d5 and d8:

Elec Config 2 1